The most significant damage to Greenpeace may be to the trust of its supporters, as 90% of funding comes from small donations. Photograph: David Mcnew/Getty Images

Greenpeace International has parted company with one of its employees after the organisation lost €3.8m (£3m) of donations last year through speculation on currency markets.

n employee at the environmental campaign group's Amsterdam headquarters took out currency exchange contracts that speculated on a weak euro in the second half of 2013, the organisation said in a statement. As a result of the euro strengthening later in the year, Greenpeace had to file record losses when closing accounts at the end of the year. Its 2013 annual report will show a budget deficit of €6.8m.

"We offer a full apology to our supporters for the series of errors that led to the loss," a statement from Greenpeace said. "We further wish to reassure people that every possible action is being taken to avoid the possibility of such a loss ever occurring again in future."

Greenpeace says the staff member, who worked in its international finance unit, was not acting for personal gain, but had failed to obtain authorisation from senior management. He has since been released from his contract.

With an annual income of €72.9m in 2013, the loss of €3.8m represents a significant hit for Greenpeace International, though the environmental group said none of its current campaigns would be affected. Greenpeace International has combined funds from more than 40 national organisations and a global budget of around €300m.

The most significant damage may be to the trust of its supporters: 90% of the organisation's funding comes from small donations of less than €100.

Greenpeace's international programme director, Pascal Husting, told the Guardian that the organisation did not generally use its funds to make investments or speculate on the stock market, but kept money in deposit accounts where it could be withdrawn quickly when needed, such as when 28 of its activists were arrested after staging a protest at a Russian offshore oil rig last year.

However, Husting said that since Greenpeace received most of its donations in euros but had to spend them in other currencies, the organisation was highly exposed to currency fluctuations and took out exchange contracts to protect its assets.

The organisation invests in property, but says this is only in cases where it is considerably cheaper to buy than to rent, such as in Moscow or London.